The present invention relates to a catalyst for the oxidation of o-xylene and/or naphthalene to phthalic anhydride, which has a plurality of catalyst zones which are arranged in series in the reaction tube and have been produced using an antimony trioxide which comprises a significant proportion of valentinite. The present invention further relates to a process for gas-phase oxidation, in which a gas stream comprising at least one hydrocarbon and molecular oxygen is passed through a catalyst produced using an antimony trioxide which comprises a significant proportion of valentinite.
Many carboxylic acids and/or carboxylic anhydrides are prepared industrially by catalytic gas-phase oxidation of hydrocarbons such as benzene, xylenes, naphthalene, toluene or durene in fixed-bed reactors. In this way, it is possible to obtain, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride. In general, a mixture of an oxygen-comprising gas and the starting material to be oxidized is passed through tubes in which a bed of a catalyst is present. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt.
Coated catalysts in which the catalytically active composition has been applied in the form of a shell to an inert support material such as steatite have been found to be useful as catalysts for these oxidation reactions. In general, the catalysts have a layer of active composition which has an essentially homogeneous chemical constitution and has been applied in the form of a shell. Furthermore, two or more different layers of active composition can be applied in succession to a support. These are then referred to as two-layer or multilayer catalysts (see, for example, DE 19839001 A1).
As catalytically active constituents of the catalytically active composition of these coated catalysts, use is generally made of titanium dioxide and vanadium pentoxide. Furthermore, small amounts of many other oxidic compounds which act as promoters to influence the activity and selectivity of the catalyst, including cesium oxide, phosphorus oxide and antimony oxide, can be present in the catalytically active composition.
Catalysts giving a particularly high PAn yield can, according to EP 1636161, be obtained when particular V2O5/Sb2O3 ratios are set and the antimony trioxide has a defined average particle size.
The presence of antimony oxides leads to an increase in the PAn selectivity; the effect is considered to be separation of the vanadium sites.
The antimony oxides used in the active composition of the catalysts can be various antimony(III), antimony(IV) and antimony(V) compounds; antimony trioxide or antimony pentoxide are usually used. EP 522871 describes the use of antimony pentoxide, US 2009/306409 and EP 1636161 disclose the use of antimony trioxide.
Compared to antimony tetroxide and antimony pentoxide, antimony trioxide has the ability to spread better on titanium dioxide, so that significantly improved distribution of the catalyst is achieved. Antimony trioxide is typically used as pure senarmontite phase (cf. Schubert, U.-A. et al., Topics in Catalysis, 2001, vol. 15(2-4), pages 195 to 200). Apart from the cubic senarmontite, there is also an orthorhombic modification of antimony trioxide, known as valentinite (Golunski, S. E. et al., Appl. Catal., 1989, vol. 48, pages 123 to 135).
There is a continual need for catalysts for gas-phase oxidations, which catalysts give a very high conversion at high selectivity.